While a patient undergoes radiotherapy treatment, the location, arrangement or shape of anatomical structures or organs can vary relative to the treatment coordinate system being used to delivery the therapy. This is especially true with respect to a planning stage, when a treatment plan is initially devised, and when they are set up for each treatment fraction (commonly referred to as interfractional motion). Furthermore, the anatomy can change during the actually treatment delivery, commonly referred to as intrafractional motion. Having the target tissue move or change shape relative to the treatment plan can cause a deterioration of the actual delivered dose to the target organs and risks exposing surrounding sensitive structures to unwanted radiation.
Due to its excellent soft-tissue contrast, ultrasound imaging is a commonly-used method to obtain images of patient anatomy throughout an entire radiotherapy process. For example, three-dimensional ultrasound (3DUS) images acquired in conjunction with a CT simulation may be used to enhance contouring of structures for planning by fused CT/3DUS images and to form an initial reference image of the internal anatomy for subsequent image guidance. Similarly, 3DUS images acquired in the treatment room, prior to each fraction, may be compared to reference 3DUS images to identify interfractional changes in internal anatomy. These 3DUS images acquired are generated by manually sweeping a 2DUS probe over a region of interest while detecting the position and orientation of the 2DUS probe. A freehand-3DUS image is then constructed relative to a room coordinate system is the imaging or therapy room. The position and orientation of the probe throughout the sweep are commonly found by detection of infrared markers affixed to the probe handle by a calibrated optical camera system.
Although freehand-3DUS imaging is useful for both planning and measuring interfractional motion, there are benefits to leaving the probe in place once the patient is set up in the treatment room, and acquiring either 2DUS or 3DUS images using a non-freehand 3DUS probe using, for example, a mechanically sweeping probe or matrix probe. Images can then be acquired at will, independent of the radiation therapist, without user variability. Multiple images can be acquired before, during, and after treatment to detect intrafractional motion, and changes in target location or shape can be compensated for during delivery of the current treatment fraction and/or in subsequent treatment fractions.
However, due to various constraints these techniques are not technically feasible. One constraint arises from the geometry of treatment and imaging apparatus; an ultrasound probe, for example, generally must maintain contact with the patient while the radiation is being delivered, but in most cases, the probe sits in the path of commonly used beam angles. This will affect the radiation dose distribution inside the patient, which in turn cannot be compensated for standard radiotherapy beams.
Moreover, radiation beams are typically directed at the patient from multiple intersecting directions, and the probe must not be in the path of the radiation beam, which would cause attenuation of the radiation, and potentially increase the skin dose to the patient. Furthermore, a user cannot remain in the radiation room to operate the ultrasound device while the beam is on due to radiation safety concerns. What is needed, therefore, are methods, systems and apparatus that facilitate the use of ultrasound imaging for planning, inter and intrafractional imaging of a patient undergoing radiotherapy treatment while not interfering with the radiation beam and permitting operation without a user in the treatment room, thus allowing for appropriate adjustments to be implemented during treatment delivery, for changes to patient positioning, or in some cases, to halt treatment altogether.